1. Field of Invention
The invention relates to a power conversion technique and more particularly to a resonant power conversion apparatus and a controlling method thereof.
2. Description of Related Art
The development trend of direct current (DC) converters, similar to that of most power supply products, is toward high efficiency, high power density, high reliability, and low costs. Since resonant converters (e.g., LLC resonant converters, etc.) are capable of performing zero-voltage switching (ZVS) in the primary winding and zero-current switching (ZCS) in the secondary winding on the full-load range, the resonant converters have gradually been applied as DC converters in recent years.
In a resonant power conversion apparatus using the resonant converter, the over-current protection is a crucial issue. In general, the resonant circuit may produce significant resonant current on the condition of over-load or short-circuit of the load. If the resonant current is not limited or protected, the resonant power conversion apparatus is very likely to be damaged by high current.
In order to provide the over-current protection to the resonant power conversion apparatus, the switching frequency of the resonant converter may be increased to raise the impedance of the resonant circuit according to the related art, and thereby the current can be limited. However, while said over-current protection mechanism is activated, the switching frequency of the resonant converter is far higher than the normal frequency, thus leading to increase of switching loss of the resonant converter. In addition, the stress tolerated by the magnetic components (e.g., capacitors or inductors) may be increased together with the increase of the switching frequency, and thus the magnetic components are more likely to be worn out.
In addition, the increasing switching frequency leads to the rising temperature of operating circuits, and hence the heat dissipation requirement for the resonant power conversion apparatus becomes rather difficult to comply with. Moreover, in order to make sure that the resonant converter may function as normal in case of high frequency, the magnetic components need be enlarged to satisfy the high frequency operational requirement.